package org.apache.lucene.search;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;
import org.apache.lucene.index.Term;
import org.apache.lucene.index.TermEnum;

/**
 * Abstract class for enumerating a subset of all terms.
 * 
 * <p>
 * Term enumerations are always ordered by Term.compareTo(). Each term in the
 * enumeration is greater than all that precede it.
 */
public abstract class FilteredTermEnum extends TermEnum {
	private Term currentTerm = null;
	private TermEnum actualEnum = null;

	public FilteredTermEnum() {
	}

	/** Equality compare on the term */
	protected abstract boolean termCompare(Term term);

	/** Equality measure on the term */
	public abstract float difference();

	/** Indicates the end of the enumeration has been reached */
	protected abstract boolean endEnum();

	protected void setEnum(TermEnum actualEnum) throws IOException {
		this.actualEnum = actualEnum;
		// Find the first term that matches
		Term term = actualEnum.term();
		if (term != null && termCompare(term))
			currentTerm = term;
		else
			next();
	}

	/**
	 * Returns the docFreq of the current Term in the enumeration. Returns -1 if
	 * no Term matches or all terms have been enumerated.
	 */
	public int docFreq() {
		if (actualEnum == null)
			return -1;
		return actualEnum.docFreq();
	}

	/** Increments the enumeration to the next element. True if one exists. */
	public boolean next() throws IOException {
		if (actualEnum == null)
			return false; // the actual enumerator is not initialized!
		currentTerm = null;
		while (currentTerm == null) {
			if (endEnum())
				return false;
			if (actualEnum.next()) {
				Term term = actualEnum.term();
				if (termCompare(term)) {
					currentTerm = term;
					return true;
				}
			} else
				return false;
		}
		currentTerm = null;
		return false;
	}

	/**
	 * Returns the current Term in the enumeration. Returns null if no Term
	 * matches or all terms have been enumerated.
	 */
	public Term term() {
		return currentTerm;
	}

	/** Closes the enumeration to further activity, freeing resources. */
	public void close() throws IOException {
		actualEnum.close();
		currentTerm = null;
		actualEnum = null;
	}
}
